Magnitude Of Earth's Magnetic Field

Magnetic field that extends from the World's outer and inner core to where it meets the solar wind

Computer simulation of Earth's field in a period of normal polarity between reversals.^[1] The lines represent magnetic field lines, blue when the field points towards the center and yellow when away. The rotation axis of Earth is centered and vertical. The dense clusters of lines are within Globe'southward core.^[two]

Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar current of air, a stream of charged particles emanating from the Sun. The magnetic field is generated by electric currents due to the move of convection currents of a mixture of molten atomic number 26 and nickel in Earth's outer cadre: these convection currents are caused by heat escaping from the core, a natural procedure called a geodynamo. The magnitude of Globe's magnetic field at its surface ranges from 25 to 65 μT (0.25 to 0.65 Grand).^[3] As an approximation, it is represented by a field of a magnetic dipole currently tilted at an angle of about xi° with respect to Earth's rotational axis, as if there were an enormous bar magnet placed at that angle through the eye of Earth. The North geomagnetic pole actually represents the Due south pole of Earth's magnetic field, and conversely the South geomagnetic pole corresponds to the northward pole of Earth'due south magnetic field (because opposite magnetic poles attract and the north end of a magnet, like a compass needle, points toward Earth'southward South magnetic field, i.e., the North geomagnetic pole near the Geographic North Pole). As of 2015, the North geomagnetic pole was located on Ellesmere Island, Nunavut, Canada.

While the North and Southward magnetic poles are usually located near the geographic poles, they slowly and continuously motility over geological time scales, but sufficiently slowly for ordinary compasses to remain useful for navigation. However, at irregular intervals averaging several hundred one thousand years, World's field reverses and the Northward and South Magnetic Poles respectively, abruptly switch places. These reversals of the geomagnetic poles leave a record in rocks that are of value to paleomagnetists in calculating geomagnetic fields in the past. Such information in turn is helpful in studying the motions of continents and ocean floors in the process of plate tectonics.

The magnetosphere is the region higher up the ionosphere that is defined by the extent of Earth's magnetic field in space. It extends several tens of thousands of kilometres into space, protecting Earth from the charged particles of the solar current of air and cosmic rays that would otherwise strip abroad the upper atmosphere, including the ozone layer that protects Earth from the harmful ultraviolet radiation.

Significance [edit]

Earth's magnetic field deflects most of the solar wind, whose charged particles would otherwise strip abroad the ozone layer that protects the Earth from harmful ultraviolet radiation.^[4] I stripping mechanism is for gas to be caught in bubbles of magnetic field, which are ripped off by solar winds.^[5] Calculations of the loss of carbon dioxide from the atmosphere of Mars, resulting from scavenging of ions by the solar wind, indicate that the dissipation of the magnetic field of Mars caused a virtually full loss of its temper.^{[6] [7]}

The written report of the past magnetic field of the Earth is known as paleomagnetism.^[8] The polarity of the Earth'south magnetic field is recorded in igneous rocks, and reversals of the field are thus detectable equally "stripes" centered on mid-ocean ridges where the sea flooring is spreading, while the stability of the geomagnetic poles between reversals has immune paleomagnetism to track the past move of continents. Reversals too provide the basis for magnetostratigraphy, a manner of dating rocks and sediments.^[9] The field likewise magnetizes the crust, and magnetic anomalies can be used to search for deposits of metal ores.^[x]

Humans have used compasses for management finding since the 11th century A.D. and for navigation since the 12th century.^[11] Although the magnetic declination does shift with time, this wandering is slow enough that a simple compass tin can remain useful for navigation. Using magnetoreception, various other organisms, ranging from some types of bacteria to pigeons, use the Earth'southward magnetic field for orientation and navigation.

Characteristics [edit]

At any location, the Earth's magnetic field can be represented by a three-dimensional vector. A typical procedure for measuring its direction is to use a compass to decide the direction of magnetic North. Its bending relative to true North is the declination ( D ) or variation. Facing magnetic North, the bending the field makes with the horizontal is the inclination ( I ) or magnetic dip. The intensity ( F ) of the field is proportional to the force it exerts on a magnet. Another common representation is in X (North), Y (Due east) and Z (Down) coordinates.^[12]

Common coordinate systems used for representing the World's magnetic field.

Intensity [edit]

The intensity of the field is often measured in gauss (G), but is generally reported in microteslas (μT), with 1 G = 100 μT. A nanotesla is also referred to equally a gamma (γ). The Globe's field ranges between approximately 25 and 65 μT (0.25 and 0.65 G).^[13] Past comparing, a potent fridge magnet has a field of about 10,000 μT (100 G).^[14]

A map of intensity contours is called an isodynamic chart. Every bit the World Magnetic Model shows, the intensity tends to subtract from the poles to the equator. A minimum intensity occurs in the Due south Atlantic Anomaly over S America while at that place are maxima over northern Canada, Siberia, and the coast of Antarctica s of Australia.^[fifteen]

The intensity of the magnetic field is subject to change over fourth dimension. A 2021 paleomagnetic study from the University of Liverpool contributed to a growing body of evidence that the World's magnetic field cycles with intensity every 200 million years. The lead author stated that "Our findings, when considered aslope the existing datasets, back up the being of an approximately 200-million-year-long bike in the strength of the World's magnetic field related to deep Earth processes."^[xvi]

Inclination [edit]

The inclination is given past an angle that can assume values between -ninety° (upwardly) to xc° (down). In the northern hemisphere, the field points downwards. It is straight downwardly at the North Magnetic Pole and rotates upward as the latitude decreases until it is horizontal (0°) at the magnetic equator. It continues to rotate upwardly until it is direct up at the Southward Magnetic Pole. Inclination can be measured with a dip circumvolve.

An isoclinic chart (map of inclination contours) for the Globe's magnetic field is shown beneath.

Declination [edit]

Declination is positive for an east deviation of the field relative to truthful due north. It tin can be estimated by comparison the magnetic north–southward heading on a compass with the direction of a angelic pole. Maps typically include information on the declination as an angle or a small diagram showing the relationship betwixt magnetic due north and truthful northward. Information on declination for a region can be represented by a chart with isogonic lines (contour lines with each line representing a fixed declination).

Geographical variation [edit]

Components of the Earth's magnetic field at the surface from the Globe Magnetic Model for 2015.^[15]

• 

  Intensity

• 

  Inclination

• 

  Declination

Dipolar approximation [edit]

Relationship between Earth's poles. A1 and A2 are the geographic poles; B1 and B2 are the geomagnetic poles; C1 (south) and C2 (north) are the magnetic poles.

Almost the surface of the Earth, its magnetic field can be closely approximated by the field of a magnetic dipole positioned at the center of the World and tilted at an bending of almost 11° with respect to the rotational centrality of the Earth.^[13] The dipole is roughly equivalent to a powerful bar magnet, with its s pole pointing towards the geomagnetic North Pole.^[17] This may seem surprising, merely the north pole of a magnet is so divers because, if immune to rotate freely, information technology points roughly n (in the geographic sense). Since the north pole of a magnet attracts the s poles of other magnets and repels the n poles, information technology must be attracted to the south pole of Earth's magnet. The dipolar field accounts for 80–90% of the field in most locations.^[12]

Magnetic poles [edit]

The motion of Earth's North Magnetic Pole across the Canadian chill.

Historically, the due north and s poles of a magnet were offset defined by the Globe's magnetic field, not vice versa, since 1 of the start uses for a magnet was as a compass needle. A magnet's N pole is defined every bit the pole that is attracted by the Earth'south Due north Magnetic Pole when the magnet is suspended and so it can plow freely. Since opposite poles attract, the North Magnetic Pole of the World is really the southward pole of its magnetic field (the identify where the field is directed downwardly into the World).^{[18] [19] [20] [21]}

The positions of the magnetic poles tin can exist divers in at least two ways: locally or globally.^[22] The local definition is the point where the magnetic field is vertical.^[23] This can exist determined by measuring the inclination. The inclination of the Globe'due south field is 90° (downwardly) at the North Magnetic Pole and -90° (upward) at the Southward Magnetic Pole. The two poles wander independently of each other and are not directly opposite each other on the globe. Movements of up to 40 kilometres (25 mi) per year have been observed for the North Magnetic Pole. Over the last 180 years, the Due north Magnetic Pole has been migrating northwestward, from Cape Adelaide in the Boothia Peninsula in 1831 to 600 kilometres (370 mi) from Resolute Bay in 2001.^[24] The magnetic equator is the line where the inclination is zero (the magnetic field is horizontal).

The global definition of the World's field is based on a mathematical model. If a line is drawn through the center of the Earth, parallel to the moment of the all-time-fitting magnetic dipole, the two positions where it intersects the Globe's surface are called the N and South geomagnetic poles. If the Earth'south magnetic field were perfectly dipolar, the geomagnetic poles and magnetic dip poles would coincide and compasses would point towards them. However, the Earth's field has a significant not-dipolar contribution, so the poles do non coincide and compasses practise not by and large point at either.

Magnetosphere [edit]

An artist'due south rendering of the construction of a magnetosphere. 1) Bow shock. ii) Magnetosheath. three) Magnetopause. 4) Magnetosphere. 5) Northern tail lobe. half-dozen) Southern tail lobe. 7) Plasmasphere.

World's magnetic field, predominantly dipolar at its surface, is distorted further out by the solar wind. This is a stream of charged particles leaving the Sun'southward corona and accelerating to a speed of 200 to 1000 kilometres per 2d. They carry with them a magnetic field, the interplanetary magnetic field (IMF).^[25]

The solar current of air exerts a pressure, and if it could reach Earth'due south temper it would erode it. However, information technology is kept away by the force per unit area of the Earth'due south magnetic field. The magnetopause, the surface area where the pressures balance, is the boundary of the magnetosphere. Despite its proper name, the magnetosphere is asymmetric, with the sunward side being nearly 10 Earth radii out only the other side stretching out in a magnetotail that extends beyond 200 World radii.^[26] Sunward of the magnetopause is the bow daze, the area where the solar wind slows abruptly.^[25]

Inside the magnetosphere is the plasmasphere, a donut-shaped region containing low-energy charged particles, or plasma. This region begins at a height of threescore km, extends up to 3 or iv World radii, and includes the ionosphere. This region rotates with the Earth.^[26] There are also 2 concentric tire-shaped regions, called the Van Allen radiation belts, with loftier-energy ions (energies from 0.1 to 10 MeV). The inner belt is 1–2 Globe radii out while the outer belt is at 4–7 Earth radii. The plasmasphere and Van Allen belts take partial overlap, with the extent of overlap varying profoundly with solar activity.^[27]

As well every bit deflecting the solar wind, the Earth's magnetic field deflects cosmic rays, high-energy charged particles that are by and large from outside the Solar Organisation. Many cosmic rays are kept out of the Solar System by the Sun's magnetosphere, or heliosphere.^[28] Past dissimilarity, astronauts on the Moon risk exposure to radiation. Anyone who had been on the Moon's surface during a peculiarly violent solar eruption in 2005 would have received a lethal dose.^[25]

Some of the charged particles do get into the magnetosphere. These spiral effectually field lines, billowy back and along between the poles several times per second. In addition, positive ions slowly drift due west and negative ions drift eastward, giving rise to a ring current. This current reduces the magnetic field at the Earth'due south surface.^[25] Particles that penetrate the ionosphere and collide with the atoms there requite rise to the lights of the aurorae and also emit X-rays.^[26]

The varying conditions in the magnetosphere, known as space weather, are largely driven by solar activity. If the solar wind is weak, the magnetosphere expands; while if information technology is strong, it compresses the magnetosphere and more of it gets in. Periods of particularly intense activity, called geomagnetic storms, tin occur when a coronal mass ejection erupts above the Sun and sends a shock wave through the Solar Organisation. Such a wave tin take simply two days to reach the Earth. Geomagnetic storms can cause a lot of disruption; the "Halloween" storm of 2003 damaged more than a tertiary of NASA'southward satellites. The largest documented storm, the Carrington Consequence, occurred in 1859. Information technology induced currents stiff enough to disrupt telegraph lines, and aurorae were reported as far southward as Hawaii.^{[25] [29]}

Time dependence [edit]

Short-term variations [edit]

Background: a set of traces from magnetic observatories showing a magnetic storm in 2000.
Globe: map showing locations of observatories and contour lines giving horizontal magnetic intensity in μ T.

The geomagnetic field changes on time scales from milliseconds to millions of years. Shorter time scales more often than not arise from currents in the ionosphere (ionospheric dynamo region) and magnetosphere, and some changes tin exist traced to geomagnetic storms or daily variations in currents. Changes over time scales of a year or more mostly reverberate changes in the Earth'due south interior, particularly the iron-rich core.^[12]

Oft, the Earth's magnetosphere is hitting by solar flares causing geomagnetic storms, provoking displays of aurorae. The short-term instability of the magnetic field is measured with the K-index.^[thirty]

Data from THEMIS show that the magnetic field, which interacts with the solar wind, is reduced when the magnetic orientation is aligned betwixt Sun and Globe – opposite to the previous hypothesis. During forthcoming solar storms, this could result in blackouts and disruptions in bogus satellites.^[31]

Secular variation [edit]

Estimated declination contours past year, 1590 to 1990 (click to see variation).

Strength of the axial dipole component of Earth's magnetic field from 1600 to 2020.

Changes in Earth's magnetic field on a time calibration of a year or more are referred to equally secular variation. Over hundreds of years, magnetic declination is observed to vary over tens of degrees.^[12] The animation shows how global declinations have changed over the last few centuries.^[32]

The direction and intensity of the dipole change over time. Over the last 2 centuries the dipole forcefulness has been decreasing at a rate of about 6.3% per century.^[12] At this rate of subtract, the field would be negligible in about 1600 years.^[33] However, this strength is about average for the concluding seven thou years, and the electric current rate of modify is non unusual.^[34]

A prominent characteristic in the non-dipolar function of the secular variation is a westward drift at a rate of nigh 0.two° per year.^[33] This drift is not the aforementioned everywhere and has varied over fourth dimension. The globally averaged migrate has been westward since nearly 1400 Advertising only eastward between about thou Advert and 1400 Advert.^[35]

Changes that predate magnetic observatories are recorded in archaeological and geological materials. Such changes are referred to as paleomagnetic secular variation or paleosecular variation (PSV). The records typically include long periods of small alter with occasional large changes reflecting geomagnetic excursions and reversals.^[36]

In July 2020 scientists report that analysis of simulations and a recent observational field model evidence that maximum rates of directional alter of Earth's magnetic field reached ~10° per yr – almost 100 times faster than current changes and x times faster than previously thought.^{[37] [38]}

Studies of lava flows on Steens Mountain, Oregon, indicate that the magnetic field could have shifted at a rate of up to 6° per day at some fourth dimension in Earth'due south history, which significantly challenges the popular understanding of how the Globe's magnetic field works.^[39] This finding was later attributed to unusual stone magnetic properties of the lava menses nether study, not rapid field change, by one of the original authors of the 1995 study.^[twoscore]

Magnetic field reversals [edit]

Geomagnetic polarity during the late Cenozoic Era. Night areas announce periods where the polarity matches today'south polarity, light areas announce periods where that polarity is reversed.

Although more often than not Globe'due south field is approximately dipolar, with an axis that is most aligned with the rotational axis, occasionally the Northward and South geomagnetic poles trade places. Bear witness for these geomagnetic reversals can be found in basalts, sediment cores taken from the ocean floors, and seafloor magnetic anomalies.^[41] Reversals occur near randomly in time, with intervals between reversals ranging from less than 0.1 million years to as much as 50 million years. The most recent geomagnetic reversal, called the Brunhes–Matuyama reversal, occurred about 780,000 years agone.^{[24] [42]} A related phenomenon, a geomagnetic circuit, takes the dipole centrality beyond the equator and then back to the original polarity.^{[43] [44]} The Laschamp outcome is an example of an excursion, occurring during the last ice age (41,000 years ago).

The past magnetic field is recorded mostly by strongly magnetic minerals, particularly iron oxides such as magnetite, that can bear a permanent magnetic moment. This remanent magnetization, or remanence, can exist acquired in more than one way. In lava flows, the direction of the field is "frozen" in small minerals as they absurd, giving rising to a thermoremanent magnetization. In sediments, the orientation of magnetic particles acquires a slight bias towards the magnetic field as they are deposited on an body of water floor or lake bottom. This is called detrital remanent magnetization.^[8]

Thermoremanent magnetization is the principal source of the magnetic anomalies around mid-ocean ridges. As the seafloor spreads, magma wells up from the mantle, cools to form new basaltic crust on both sides of the ridge, and is carried away from it past seafloor spreading. Every bit it cools, it records the direction of the Earth'due south field. When the Earth's field reverses, new basalt records the reversed direction. The result is a series of stripes that are symmetric about the ridge. A ship towing a magnetometer on the surface of the ocean can notice these stripes and infer the age of the bounding main floor below. This provides information on the rate at which seafloor has spread in the past.^[viii]

Radiometric dating of lava flows has been used to establish a geomagnetic polarity time scale, part of which is shown in the prototype. This forms the basis of magnetostratigraphy, a geophysical correlation technique that can be used to date both sedimentary and volcanic sequences as well as the seafloor magnetic anomalies.^[8]

Earliest appearance [edit]

Paleomagnetic studies of Paleoarchean lava in Commonwealth of australia and conglomerate in Due south Africa accept concluded that the magnetic field has been present since at least about 3,450 1000000 years ago.^{[45] [46] [47]}

Future [edit]

Variations in virtual axial dipole moment since the last reversal.

Starting in the late 1800s and throughout the 1900s and later, the overall geomagnetic field has become weaker; the present strong deterioration corresponds to a 10–15% decline and has accelerated since 2000; geomagnetic intensity has declined almost continuously from a maximum 35% to a higher place the modern value, from circa year i Advertizing. The charge per unit of subtract and the current strength are within the normal range of variation, equally shown by the record of by magnetic fields recorded in rocks.

The nature of Earth's magnetic field is ane of heteroscedastic (seemingly random) fluctuation. An instantaneous measurement of it, or several measurements of it across the bridge of decades or centuries, are not sufficient to extrapolate an overall trend in the field force. It has gone up and down in the past for unknown reasons. Also, noting the local intensity of the dipole field (or its fluctuation) is bereft to characterize Earth's magnetic field as a whole, equally information technology is non strictly a dipole field. The dipole component of Earth'due south field can diminish even while the full magnetic field remains the same or increases.

The Earth's magnetic north pole is globe-trotting from northern Canada towards Siberia with a shortly accelerating charge per unit—ten kilometres (6.2 mi) per year at the beginning of the 1900s, up to 40 kilometres (25 mi) per year in 2003,^[24] and since so has but accelerated.^{[48] [49]}

Physical origin [edit]

World's core and the geodynamo [edit]

The Globe'south magnetic field is believed to exist generated by electric currents in the conductive iron alloys of its core, created by convection currents due to heat escaping from the core.

A schematic illustrating the relationship between motion of conducting fluid, organized into rolls by the Coriolis strength, and the magnetic field the motility generates.^[50]

The Earth and most of the planets in the Solar System, every bit well equally the Sun and other stars, all generate magnetic fields through the motility of electrically conducting fluids.^[51] The Globe's field originates in its core. This is a region of atomic number 26 alloys extending to about 3400 km (the radius of the World is 6370 km). Information technology is divided into a solid inner cadre, with a radius of 1220 km, and a liquid outer core.^[52] The motility of the liquid in the outer core is driven by heat flow from the inner core, which is about 6,000 Thou (5,730 °C; 10,340 °F), to the cadre-drapery purlieus, which is most 3,800 Chiliad (3,530 °C; 6,380 °F).^[53] The estrus is generated by potential energy released past heavier materials sinking toward the core (planetary differentiation, the fe catastrophe) equally well as disuse of radioactive elements in the interior. The blueprint of flow is organized past the rotation of the Earth and the presence of the solid inner core.^[54]

The machinery by which the Globe generates a magnetic field is known as a dynamo.^[51] The magnetic field is generated by a feedback loop: current loops generate magnetic fields (Ampère's circuital law); a changing magnetic field generates an electrical field (Faraday's constabulary); and the electric and magnetic fields exert a force on the charges that are flowing in currents (the Lorentz strength).^[55] These furnishings can be combined in a partial differential equation for the magnetic field called the magnetic induction equation,

${\displaystyle {\frac {\partial \mathbf {B} }{\fractional t}}=\eta \nabla ^{2}\mathbf {B} +\nabla \times (\mathbf {u} \times \mathbf {B} ),}$

where u is the velocity of the fluid; B is the magnetic B-field; and η=i/σμ is the magnetic diffusivity, which is inversely proportional to the product of the conductivity σ and the permeability μ .^[56] The term ∂B/∂t is the time derivative of the field; ∇^two is the Laplace operator and ∇× is the curl operator.

The kickoff term on the right hand side of the induction equation is a diffusion term. In a stationary fluid, the magnetic field declines and whatsoever concentrations of field spread out. If the World's dynamo shut off, the dipole part would disappear in a few tens of thousands of years.^[56]

In a perfect conductor ( ${\displaystyle \sigma =\infty \;}$ ), in that location would be no diffusion. By Lenz'south constabulary, any alter in the magnetic field would be immediately opposed by currents, so the flux through a given volume of fluid could not change. As the fluid moved, the magnetic field would go with it. The theorem describing this effect is called the frozen-in-field theorem. Even in a fluid with a finite conductivity, new field is generated by stretching field lines as the fluid moves in ways that deform it. This procedure could go on generating new field indefinitely, were information technology not that as the magnetic field increases in strength, it resists fluid move.^[56]

The motion of the fluid is sustained by convection, motion driven by buoyancy. The temperature increases towards the center of the Earth, and the higher temperature of the fluid lower downwards makes information technology buoyant. This buoyancy is enhanced by chemical separation: As the core cools, some of the molten iron solidifies and is plated to the inner core. In the process, lighter elements are left behind in the fluid, making it lighter. This is called compositional convection. A Coriolis effect, caused past the overall planetary rotation, tends to organize the catamenia into rolls aligned forth the north–south polar centrality.^{[54] [56]}

A dynamo can amplify a magnetic field, but it needs a "seed" field to become it started.^[56] For the Earth, this could have been an external magnetic field. Early on in its history the Sun went through a T-Tauri phase in which the solar wind would have had a magnetic field orders of magnitude larger than the nowadays solar wind.^[57] However, much of the field may have been screened out past the Earth's drape. An alternative source is currents in the core-curtain purlieus driven by chemical reactions or variations in thermal or electrical electrical conductivity. Such effects may still provide a small bias that are part of the boundary conditions for the geodynamo.^[58]

The average magnetic field in the Earth'due south outer core was calculated to exist 25 gauss, 50 times stronger than the field at the surface.^[59]

Numerical models [edit]

Simulating the geodynamo by computer requires numerically solving a set up of nonlinear partial differential equations for the magnetohydrodynamics (MHD) of the World's interior. Simulation of the MHD equations is performed on a 3D grid of points and the fineness of the filigree, which in role determines the realism of the solutions, is express mainly past computer power. For decades, theorists were confined to creating kinematic dynamo figurer models in which the fluid motion is chosen in accelerate and the outcome on the magnetic field calculated. Kinematic dynamo theory was mainly a affair of trying different flow geometries and testing whether such geometries could sustain a dynamo.^[60]

The offset self-consistent dynamo models, ones that decide both the fluid motions and the magnetic field, were developed by two groups in 1995, one in Nihon^[61] and ane in the United states.^{[1] [62]} The latter received attention considering it successfully reproduced some of the characteristics of the Earth's field, including geomagnetic reversals.^[sixty]

Consequence of body of water tides [edit]

The oceans contribute to Earth'south magnetic field. Seawater is an electrical conductor, and therefore interacts with the magnetic field. As the tides bicycle around the ocean basins, the ocean water essentially tries to pull the geomagnetic field lines forth. Because the salty h2o is slightly conductive, the interaction is relatively weak: the strongest component is from the regular lunar tide that happens most twice per day (M2). Other contributions come from body of water swell, eddies, and even tsunamis.^[63]

The force of the interaction depends also on the temperature of the sea water. The entire rut stored in the ocean can at present be inferred from observations of the Earth's magnetic field.^{[64] [63]}

Currents in the ionosphere and magnetosphere [edit]

Electric currents induced in the ionosphere generate magnetic fields (ionospheric dynamo region). Such a field is always generated near where the atmosphere is closest to the Sun, causing daily alterations that can deflect surface magnetic fields by every bit much equally 1°. Typical daily variations of field strength are near 25 nT (one role in 2000), with variations over a few seconds of typically effectually 1 nT (one function in 50,000).^[65]

Measurement and analysis [edit]

Detection [edit]

The Earth'south magnetic field strength was measured by Carl Friedrich Gauss in 1832^[66] and has been repeatedly measured since then, showing a relative decay of about 10% over the last 150 years.^[67] The Magsat satellite and subsequently satellites have used 3-axis vector magnetometers to probe the three-D construction of the Earth's magnetic field. The later Ørsted satellite immune a comparison indicating a dynamic geodynamo in action that appears to be giving rise to an alternate pole under the Atlantic Body of water due west of South Africa.^[68]

Governments sometimes operate units that specialize in measurement of the World'due south magnetic field. These are geomagnetic observatories, typically part of a national Geological survey, for case, the British Geological Survey'due south Eskdalemuir Observatory. Such observatories can measure and forecast magnetic conditions such equally magnetic storms that sometimes touch on communications, electrical ability, and other human being activities.

The International Real-time Magnetic Observatory Network, with over 100 interlinked geomagnetic observatories around the world, has been recording the Globe's magnetic field since 1991.

The military machine determines local geomagnetic field characteristics, in order to detect anomalies in the natural background that might be caused by a significant metal object such equally a submerged submarine. Typically, these magnetic anomaly detectors are flown in aircraft like the United kingdom's Nimrod or towed as an instrument or an assortment of instruments from surface ships.

Commercially, geophysical prospecting companies also apply magnetic detectors to identify naturally occurring anomalies from ore bodies, such as the Kursk Magnetic Anomaly.

Crustal magnetic anomalies [edit]

A model of curt-wavelength features of World'southward magnetic field, attributed to lithospheric anomalies^[69]

Magnetometers detect minute deviations in the Earth'southward magnetic field acquired by fe artifacts, kilns, some types of stone structures, and even ditches and middens in archaeological geophysics. Using magnetic instruments adapted from airborne magnetic anomaly detectors developed during Earth State of war Ii to detect submarines,^[70] the magnetic variations across the ocean floor have been mapped. Basalt — the iron-rich, volcanic stone making upward the sea floor^[71] — contains a strongly magnetic mineral (magnetite) and can locally distort compass readings. The distortion was recognized past Icelandic mariners as early as the belatedly 18th century.^[72] More of import, because the presence of magnetite gives the basalt measurable magnetic properties, these magnetic variations have provided another means to written report the deep body of water floor. When newly formed rock cools, such magnetic materials tape the Earth'due south magnetic field.^[72]

Statistical models [edit]

Each measurement of the magnetic field is at a particular identify and time. If an authentic guess of the field at some other place and fourth dimension is needed, the measurements must exist converted to a model and the model used to brand predictions.

Spherical harmonics [edit]

Schematic representation of spherical harmonics on a sphere and their nodal lines. P _{ℓ 1000} is equal to 0 along yard great circles passing through the poles, and forth ℓ-m circles of equal breadth. The function changes sign each ℓtime it crosses one of these lines.

Example of a quadrupole field. This can likewise be constructed by moving two dipoles together.

The most common way of analyzing the global variations in the Globe'due south magnetic field is to fit the measurements to a prepare of spherical harmonics. This was first washed by Carl Friedrich Gauss.^[73] Spherical harmonics are functions that oscillate over the surface of a sphere. They are the product of two functions, one that depends on latitude and 1 on longitude. The function of longitude is zero forth nix or more great circles passing through the North and South Poles; the number of such nodal lines is the absolute value of the order m . The function of latitude is zero along nix or more breadth circles; this plus the social club is equal to the degree ℓ. Each harmonic is equivalent to a particular arrangement of magnetic charges at the eye of the Globe. A monopole is an isolated magnetic accuse, which has never been observed. A dipole is equivalent to two opposing charges brought close together and a quadrupole to two dipoles brought together. A quadrupole field is shown in the lower figure on the right.^[12]

Spherical harmonics tin can represent whatever scalar field (function of position) that satisfies sure properties. A magnetic field is a vector field, only if it is expressed in Cartesian components 10, Y, Z , each component is the derivative of the same scalar function chosen the magnetic potential. Analyses of the World's magnetic field employ a modified version of the usual spherical harmonics that differ by a multiplicative gene. A least-squares fit to the magnetic field measurements gives the Earth's field as the sum of spherical harmonics, each multiplied by the all-time-fitting Gauss coefficient g_g ^ℓ or h_m ^ℓ .^[12]

The lowest-degree Gauss coefficient, g ₀ ⁰ , gives the contribution of an isolated magnetic charge, so it is nada. The next 3 coefficients – g ₁ ⁰ , chiliad ₁ ¹ , and h ₁ ¹ – decide the direction and magnitude of the dipole contribution. The best fitting dipole is tilted at an bending of about ten° with respect to the rotational centrality, equally described earlier.^[12]

Radial dependence [edit]

Spherical harmonic assay tin can exist used to distinguish internal from external sources if measurements are available at more than i superlative (for example, basis observatories and satellites). In that case, each term with coefficient one thousand_m ^ℓ or h_yard ^ℓ can be carve up into 2 terms: one that decreases with radius as 1/r ^ℓ+1 and i that increases with radius as r ^ℓ . The increasing terms fit the external sources (currents in the ionosphere and magnetosphere). Nonetheless, averaged over a few years the external contributions average to nil.^[12]

The remaining terms predict that the potential of a dipole source (ℓ=ane) drops off equally 1/r ⁱⁱ . The magnetic field, existence a derivative of the potential, drops off equally ane/r ⁱⁱⁱ . Quadrupole terms drop off as 1/r ⁴ , and higher order terms drop off increasingly chop-chop with the radius. The radius of the outer cadre is about half of the radius of the Earth. If the field at the core-mantle purlieus is fit to spherical harmonics, the dipole part is smaller by a factor of about 8 at the surface, the quadrupole part by a factor of 16, so on. Thus, only the components with large wavelengths can be noticeable at the surface. From a variety of arguments, it is usually causeless that only terms up to caste 14 or less have their origin in the core. These have wavelengths of about 2,000 km (1,200 mi) or less. Smaller features are attributed to crustal anomalies.^[12]

Global models [edit]

The International Association of Geomagnetism and Aeronomy maintains a standard global field model called the International Geomagnetic Reference Field (IGRF). It is updated every five years. The 11th-generation model, IGRF11, was developed using data from satellites (Ørsted, Champ and SAC-C) and a globe network of geomagnetic observatories.^[74] The spherical harmonic expansion was truncated at degree x, with 120 coefficients, until 2000. Subsequent models are truncated at degree 13 (195 coefficients).^[75]

Another global field model, called the Globe Magnetic Model, is produced jointly by the The states National Centers for Environmental Information (formerly the National Geophysical Data Middle) and the British Geological Survey. This model truncates at degree 12 (168 coefficients) with an judge spatial resolution of iii,000 kilometers. It is the model used by the United states Department of Defense, the Ministry of Defence (Britain), the Us Federal Aviation Assistants (FAA), the North Atlantic Treaty Organization (NATO), and the International Hydrographic Organisation every bit well as in many civilian navigation systems.^[76]

The above models just take into account the "master field" at the core-curtain boundary. Although generally practiced plenty for navigation, higher-accurateness use cases require smaller-calibration magnetic anomalies and other variations to be considered. Some examples are (encounter geomag.united states of america ref for more):^[77]

• The "comprehensive modeling" (CM) appproach by the Goddard Space Flight Center (NASA and GSFC) and the Danish Space Research Institute. CM attempts to reconcile data with profoundly varying temporal and spatial resolution from footing and satellite sources. The latest version every bit of 2022 is CM5 of 2016. It provides separate components for chief field plus lithosphere (crustal), M2 tidal, and chief/induced magnetosphere/ionosphere variations.^[78]
• The US National Centers for Environmental Information adult the Enhanced Magnetic Model (EMM), which extends to degree and gild 790 and resolves magnetic anomalies down to a wavelength of 56 kilometers. It was compiled from satellite, marine, aeromagnetic and ground magnetic surveys. As of 2018^[update], the latest version, EMM2017, includes data from The European Space Agency'south Swarm satellite mission.^[79]

For historical data almost the primary field, the IGRF may be used back to year 1900.^[75] A specialized GUFM1 model estimates back to year 1590 using transport's logs.^[fourscore] Paleomagnetic inquiry has produced models dating back to 10,000 BCE.^[81]

Biomagnetism [edit]

Animals, including birds and turtles, tin can notice the Earth'due south magnetic field, and use the field to navigate during migration.^[82] Some researchers have constitute that cows and wild deer tend to marshal their bodies north–south while relaxing, only non when the animals are under high-voltage power lines, suggesting that magnetism is responsible.^{[83] [84]} Other researchers reported in 2011 that they could non replicate those findings using dissimilar Google Earth images.^[85]

Very weak electromagnetic fields disrupt the magnetic compass used by European robins and other songbirds, which utilise the Earth'due south magnetic field to navigate. Neither ability lines nor cellphone signals are to blame for the electromagnetic field consequence on the birds;^[86] instead, the culprits have frequencies between 2 kHz and five MHz. These include AM radio signals and ordinary electronic equipment that might be establish in businesses or individual homes.^[87]

See also [edit]

• Geomagnetic jerk
• Geomagnetic latitude
• Magnetic field of Mars
• Magnetotellurics
• Operation Argus

References [edit]

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Farther reading [edit]

• Campbell, Wallace H. (2003). Introduction to geomagnetic fields (2nd ed.). New York: Cambridge Academy Printing. ISBN978-0-521-52953-two.
• Gramling, Carolyn (1 February 2019). "Earth's core may accept hardened but in time to save its magnetic field". Scientific discipline News . Retrieved three Feb 2019.
• Herndon, J. 1000. (1996-01-23). "Substructure of the inner core of the Earth". PNAS. 93 (2): 646–648. Bibcode:1996PNAS...93..646H. doi:ten.1073/pnas.93.2.646. PMC40105. PMID 11607625.
• Hollenbach, D. F.; Herndon, J. M. (2001-09-25). "Deep-World reactor: Nuclear fission, helium, and the geomagnetic field". PNAS. 98 (xx): 11085–90. Bibcode:2001PNAS...9811085H. doi:x.1073/pnas.201393998. PMC58687. PMID 11562483.
• Beloved, Jeffrey J. (2008). "Magnetic monitoring of Globe and space" (PDF). Physics Today. 61 (two): 31–37. Bibcode:2008PhT....61b..31H. doi:x.1063/1.2883907.
• Merrill, Ronald T. (2010). Our Magnetic Earth: The Science of Geomagnetism. University of Chicago Press. ISBN978-0-226-52050-half-dozen.
• Merrill, Ronald T.; McElhinny, Michael Westward.; McFadden, Phillip L. (1996). The magnetic field of the earth: paleomagnetism, the core, and the deep pall. Academic Press. ISBN978-0-12-491246-5.
• "Temperature of the Earth'south cadre". NEWTON Inquire a Scientist. 1999. Archived from the original on 2010-09-08. Retrieved 2006-01-21 .
• Tauxe, Lisa (1998). Paleomagnetic Principles and Do. Kluwer. ISBN978-0-7923-5258-vii.
• Towle, J. Northward. (1984). "The Anomalous Geomagnetic Variation Field and Geoelectric Structure Associated with the Mesa Butte Error Organization, Arizona". Geological Order of America Bulletin. 9 (2): 221–225. Bibcode:1984GSAB...95..221T. doi:10.1130/0016-7606(1984)95<221:TAGVFA>2.0.CO;2.
• Turner, Gillian (2011). Northward Pole, South Pole: The epic quest to solve the smashing mystery of Earth'southward magnetism. New York, NY: The Experiment. ISBN9781615190317.
• Wait, James R. (1954). "On the relation betwixt telluric currents and the earth's magnetic field". Geophysics. xix (2): 281–289. Bibcode:1954Geop...19..281W. doi:10.1190/one.1437994. S2CID 51844483.
• Walt, Martin (1994). Introduction to Geomagnetically Trapped Radiations. Cambridge University Press. ISBN978-0-521-61611-nine.

External links [edit]

• Geomagnetism & Paleomagnetism background material Archived 2013-03-03 at the Wayback Motorcar. American Geophysical Marriage Geomagnetism and Paleomagnetism Section.
• National Geomagnetism Program. United states Geological Survey, March 8, 2011.
• BGS Geomagnetism. Information on monitoring and modeling the geomagnetic field. British Geological Survey, August 2005.
• William J. Wide, Will Compasses Point South?. The New York Times, July xiii, 2004.
• John Roach, Why Does Earth's Magnetic Field Flip?. National Geographic, September 27, 2004.
• Magnetic Storm. PBS NOVA, 2003. (ed. nigh pole reversals)
• When N Goes South. Projects in Scientific Computing, 1996.
• The Swell Magnet, the Earth, History of the discovery of Earth's magnetic field past David P. Stern.
• Exploration of the Earth's Magnetosphere Archived 2013-02-fourteen at the Wayback Machine, Educational web site past David P. Stern and Mauricio Peredo
• International Geomagnetic Reference Field 2011
• Global evolution/anomaly of the Earth's magnetic field Archived 2016-06-24 at the Wayback Automobile Sweeps are in 10° steps at x years intervals. Based on information from: The Institute of Geophysics, ETH Zurich Archived 2007-10-31 at the Wayback Machine
• Patterns in Earth's magnetic field that evolve on the order of one,000 years Archived 2018-07-20 at the Wayback Motorcar. July 19, 2017
• Chree, Charles (1911). "Magnetism, Terrestrial". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 17 (11th ed.). Cambridge University Press. pp. 353–385. (with dozens of tables and several diagrams)

Magnitude Of Earth's Magnetic Field,

Source: https://en.wikipedia.org/wiki/Earth%27s_magnetic_field

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